Electrodeposition of an iridescent chromium coating

ABSTRACT

THIS INVENTION RELATES TO NOVEL COMPOSITIONS AND TO A PROCESS OF PREPARING AN IRIDESCENT CHROMIUM PLATED ARTICLE WHICH COMPRISES PASSING CURRENT FROM AN ANODE TO A METAL CATHODE THROUGH AN AQUEOUS CHROMIUM PLATING SOLUTION WHICH CONTAINS AT LEAST ONE CHROMIUM COMPOUND PROVIDING HEXAVALENT CHROMIUM METAL IONS FOR ELECTROPLATING CHROMIUM AND: (A) AS FIRST ADDITIVE COMPOSITION AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF AN ALKYL SULFONIC ACID, A HALOALKYL SULFONIC ACID, AN AKYL PHOSPHONIC ACID, A HALOALKYL PHOSPHONIC ACID, AND SATS THEREOF; AND (B) AS SECOND ADDITIVE COMPOSITION AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF CARBOXYLIC ACIDS OF FROM 1-3 CARBON ATOMS, HALOGEN SUBSTITUTED CARBOXYLIC ACIDS OF FROM 1-3 CARBON ATOMS, AND SALTS THEREOF; TO PRODUCE AN IRIDESCENT CHROMIUM SURFACE ON SAID ARTICLE.

United States Patent Office 3,745,097 Patented July 10, 1973 3,745,097 ELECTRODEPOSITION OF AN IRIDESCENT CHROMIUM COATING Hyman Chessin and Paul D. Walker, Birmingham, Mich., assignors to M & T Chemicals Inc New York, N.Y. No Drawing. Filed May 26, 1969, Ser. No. 827,947

Int. Cl. C23b 5/06 U.S. Cl. 20451 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to novel compositions and to a process of preparing an iridescent chromium plated article which comprises passing current from an anode to a metal cathode through an aqueous chromium plating solution which contains at least one chromium compound providing hexavalent chromium metal ions for electroplating chromium and:

to produce an iridescent chromium surface on said article.

This invention relates to a novel decorative metal coating, to bath compositions used to prepare such coatings, and to processes for the preparation of such decorative metal coatings. More particularly, this invention relates to decorative electrodeposited chromium surfaces which have an iridescent appearance when exposed to natural or artificial light.

In accordance with certain of its aspects, this invention relates to novel compositions and to a process of preparing an iridescent chromium plated article which comprises passing current from an anode to a metal cathode through an aqueous chromium plating solution which contains at least one chromium compound providing hexavalent chromium metal ions for electroplating chromium and:

(a) as first additive composition at least one compound selected from the group consisting of an alkyl sulfonic acid, a haloalkyl sulfonic acid, an alkyl phosphonic acid, a haloalkyl phosphonic acid, and salts thereof; and

(b) as second additive composition at least one member selected from the group consisting of carboxylic acids of from 1-3 carbon atoms, halogen substituted carboxylic acids of from l-3 carbon atoms, and salts thereof;

to produce an iridescent chromium surface on said article.

The basis material which may be treated according to this invention to produce an iridescent chromium deposit may include a basis material such as iron, steel, brass, bronze, copper, zinc, aluminum, magnesium, nickel, etc. either pure or in the form of an alloy or as multilayer deposits. The preferred basis metal to be plated in accordance with this invention may be steel, zinc, or brass and most preferably steel, zinc, or brass which has first been plated with a conductive deposit such as a plate of bright nickel, typically preceded by a first plate of copper, bronze, or semi-bright nickel.

Other basis materials to be treated by the process of this invention may include plastics and resins including acrylonitrile-butadiene-styrene, acetals, acrylics, alkyds, alkyls, amines, cellulosics, chlorinated polyethers, epoxies, furanes, fluorocarbons, isocyanates (urethanes), polyamides (nylons), phenoxides, phenolics, polycarbonates, vinyls, and copolymers, etc. of these materials. When the basis material to be treated by the process of this invention is a plastic or resin, the surface thereof will be treated by deposition thereon of a conductive deposit. such as a nickel deposit.

The basis material bearing a conductive surface, preferably a bright nickel plate, may be immediately treated after disposition of such plate or it may be water rinsed; or it may be rinsed, dipped in aqueous acid solution such as 0.5%l0%, say 2%, by weight of sulfuric acid prior to further treatment. The so treated material may be dried or it may be further treated as is. If drying has been permitted, the conductive surface may be cleaned as by cathodically treating in alkaline cleaner followed by rinsing in water or dipping in an acid solution before further treatment.

The aqueous chromium plating solution employed according to the invention may be prepared from commercial chromic acid solutions. Preferably, a portion of the chromic acid used in the bath composition may be replaced by a salt of hexavalent chromium such as sodium chromate, sodium dichromate, potassium chromate, potassium dichromate, strontium chromate, magnesium dichromate, lanthanum chromate, lanthanum dichromate, etc. In many cases it may be advantageous to employ one or more of such salts in order to provide for a means of control of the composition or of the acidity of the aqueous chromium plating bath.

Catalyst anions may also be included in the aqueous chromium plating bath to regulate the concentrations of the active ingredients during electroplating. 'For example, sulfate, fluoride, and complex fluorides may be used. Typically, fluosilicates, fluoborates, fiuoaluminates, fluotitanates, and fiuozirconates may be included in the aqueous chromium plating bath composition without adversely affecting the iridescent light reflective properties of the novel decorative chromium electroplate which is produced according to the invention.

Suitable neutralizers and/or bufiers may also be employed in the bath composition of the invention. Examples of such neutralizers or buffers may include phosphates, borates, sulfonates, and carboxylates. In addition, catalyst anions as heretofore recited may also be used in amounts sufficient to act as buffers or neutralizers in combined eifect with their catalyst properties.

The alkyl sulfonic acids, haloalkyl sulfonic acids, alkyl phosphonic acids, haloalkyl phosphonic acids, and salts thereof which may be used according to the invention may contain at least one carbon atom and, typically, from 13 carbon atoms. Most preferred alkyl sulfonic acids and alkyl phosphonic acids (including salts thereof) contain short chain alkyl groups of 1-3 carbon atoms. Typically halogen substitutents are chlorine, bromine, and fluorine.

Alkyl sulfonic acids, haloalkyl sulfonic acids, and salts thereof which may be employed according to the invention to produce iridescent chromium deposits include compounds of the formula wherein a is an integer of 1-3 (representing the valence of the organic group R) and R is an organic group containing 1-3 carbon atoms, typically an alkyl, alkene, haloalkyl, haloalkylene, or trivalent group of 1-3 carbon atoms. Typical groups include methyl, ethyl-n-propyl, i-propyl, chloromethyl, dichloromethyl, trichloromethyl,

i1,2-dibromoethyl ,and 1,2-difluoro n-propyl. R is preferably a group containing 1-2 carbon atoms. Thus, suitable materials include methyl sulfonic acid, ethyl sulfonic acid, npropyl sulfonic acid, chloromethyl sulfuric acid, chlorobromoethyl sulfonic acid, 1,1,2-tribromo-n-propyl sulfonic acid, 1,2-ethyldisulfonic acid, and salts thereof may be used. Examples of suitable salts of the alkyl sulfonic acids and haloalkyl sulfonic acids include alkali metal salts (e.g. Na, K, Li, Cs), ammonium salts, and Ca, Sr, Mg, etc. salts. The alkyl sulfonic acid salt or haloalkyl sulfonic acid salt may be any bath compatible salt (including esters which hydrolyze to the acid) or mixtures of such salts and/or alkyl sulfonic acids. The most preferred sulfonic acids include methyl sulfonic acid, ethyl sulfonic acid, and 1,2-ethyldisulfonic acid and salts thereof.

The alkyl phosphonic acids, haloalkyl phosphonic acids, and salts thereof which may be used according to the invention to produce iridescent chromium deposits may include compounds of the formula wherein b is an integer of 1-3 (representing the valence of the organic group R) and R' is an alkyl, alkylene, haloalkylene, haloalkyl or trivalent group of 1-3 carbon atoms. Typical R groups include methyl, ethyl, n-propyl, i-propyl, chloromethyl, dichloromethyl, trichloromethyl, 1,2-dibrornoethyl, 1,2-difluoro-n-propyl,

etc. R' is preferably a group containing 1-3 carbon atoms.

Acids such as mcthylphosphonic acid, dibromoethylphosphonic acid, ethylphosphonic acid, 1,2-dichloroethyl phosphonic acid, n-propylphosphonic acid, trichloro-npropylphosphonic acid, i-propylphosphonic acid, trifluoroethylphosphonic acid, and salts thereof may be used. Examples of suitable salts of the alkyl phosphonic acids and haloalkyl phosphonic acidsinclude alkali metal salts (e.g. Na, K, Li, Cs), ammonium salts, and Ca, Sr, Mg, etc. salts. The alkyl phosphonic acid salt or haloalkyl phosphonic acid salt may be any bath compatible salt (including esters which hydrolyze to the acid) or mixtures of such salts and/or alkyl phosphonic acids. Most preferred phosphonic acids include chloro-methyl phosphonic acid and methyl phosphonic acid.

The total amount of sulfonic and/or phosphonic acids (including salts thereof) which may be employed may be 1-50 g./1. of total bath solution, typically 3-20 g./l., and preferably 4-8 g./l.

The carboxylic acids (including salts thereof) which may be used in combinatioin with the sulfonic and phosphonic acid materials of the invention to produce an iridescent chromium plated article include acids having the formula r z-o wherein c is an integer of 1-3 representing the number of carboxyl groups in the molecule, and Z is a hydrogen atom or an organic group of 1-3 carbon atoms. Thus, Z may be a monovalent, divalent, or trivalent group depending upon the number of carboxyl groups attached thereto. Z is preferably a group containing 1-2 carbon atoms. Typical Z groups include alkyl, alkylene, haloalkyl, haloalkylene, and trivalent groups containing 1-3 carbon atoms. Typical carboxylic acids which may be used in combination with the sulfonic and/or phosphonic acids to produce the iridescent chromium deposits of the invention include acetic acid, succinic acid, chloroacetic acid, 1,3,5- tricarboxypentane, and trifluoroacetic acid.

Any weak substituted or unsubstituted buffering carboxylic acid (including salts thereof) wherein each carboxylate group is associated with no more than two additional substituted or unsubstituted carbon atoms may be employed according to the invention. Partially esterified or fully esterified acids may be used in combination with salts to obtain the desired iridescent effects described herein.

The amount of carboxylic acids (including salts and esters thereof which hydrolyze to the acid) which may be used according to the invention in combination with the sulfonic and/or phosphonic acids (including salts thereof) may vary from 0.5 g./l. to 50 g./l. of bath compositions. Typical concentrations which may be employed includes 5 g./l.-4O g./l. and preferably 8 g./l.20 g./ 1.

Suitable ranges of components which may be employed according to the invention include the following:

TABLE I All amounts in grams per liter {g.ll.)

Typical Preferred Optimum Sulfonic acid 1-50 320 4-8 Phosphonic acid 1-50 2-20 3-6 Carboxylic acid 0. 5-50 5-40 8-20 All amounts in grams per liter (g./1.) Chromic acid (CrO -600 Sulfate (SO 0.6-6 Methylsulfonic acid 2-6 Acetic acid 10-25 The iridescent chromium deposit may be obtained using plating currents of 2-30 amperes per square decimeter (a.s.d.) at temperatures of 10-80" C., using plating times of one-half minute to one hour (typically 2-30 minutes).

The iridescent chromium plated articles prepared using the compositions of the invention may contain mounds of chromium deposits having a mean height above the surface of 0.025-5 microns wherein said mounds are positioned in adjacent and semi-overlapping strings, said strings being positioned in approximately parallel configurations to form an iridescent chromium surface havnig about 70- 1500 strings per millimeter and 6 10= -6 10 mounds of chromium deposits per square centimeter of surface. Such iridescent chromium plated articles may subsequently be given a supplementary metallic coating of metals such as nickel, copper, chromium, etc. to enhance or modify the color or reflectivity of the iridescent surface. Similarly, organic or inorganic coatings may be used to vary the decorative rainbow effects on all or a portion of said surfaces.

The mounds of chromium deposits are roughly circular in cross section and may have a curvature approaching that of a sphere or a portion of a sphere. The mean height of a mound of chromium deposit is measured from the plane which joins the totality of points representing the average of the minimum and maximum of the surface upon which said mounds are positioned. Microscopic examination of said surface shows that the mounds of deposit may be completely separate with distances between said mounds of up to three or four mound diameters. The mounds may touch or overlap to form continuous strings or semi-continuous strings, said strings being positioned in approximately parallel configurations analogous to strings of beads placed upon and conforming to the shape of a given surface. It is believed that the parallel configurations (which are not necessarily linear) produce an effect such as may be obtained from a diffraction grating using parallel lines. It has been found that particularly striking iridescent effects may be obtained from an iridescent chromium plated article wherein the article contains about 6 10 -6- 10 mounds per square centimeter having an average height of 0.025-5 microns, wherein the mounds are positioned on the surface of said iridescent chromium plated article so that a series of continuous lines connecting the centers of said mounds produces a network of lines or lattice and said network of lines or lattice contains at least 790 parallel lines per linear centimeter of length as measured by intersecting points on a straight line drawn in any direction for a length of one centimeter from the center of a given mound of iridescent chromium plated surface. Thus, a line of one centimeter length drawn from any given mound on the chromium plated surface of the iridescent chromium plated article of the invention intersects at least about 790 approximately parallel strings of mounds when such strings of mounds are connected by lines drawn to the centers of the mounds comprising said approximately parallel strings.

Bright iridescent chromium plated articles may be characterized as having a surface containing mounds of chromium deposits wherein the average height of the mounds is about 0.075-2.5 microns and wherein the average space between any two approximately parallel adjacent strings of mounds (as measured by the average distance between the two lines joining the center of the mounds of each string) is approximately 2.9-3.0 microns.

The following examples are submitted for the purpose of illustration only and are not to be construed as limiting the scope of the invention in any way.

In all of the examples an iridescent span value of at least millimeters indicates production of a satisfactory iridescent product, a value of at least 30-40 millimeters indicates excellent iridescent properties, and a value greater than 50-60 indicates outstanding results.

EXAMPLE 1 Component: Grams per liter (g./l.) Chromic acid (CrO 330.0

Sulfate (SO 1.32 Methylsulfonic acid 4.2 Acetic acid 16.0

A panel of buffed brass measuring 75 x 100 x 0.45 millimeters was cleaned and plated with a bright nickel deposit. The nickel plated panel was immersed in a Hull Cell so that the total immersed area in the plating solution measured 50 x 100 millimeters after addition of the solution. The nickel plated panel was connected as cathode and a total current of 8 amperes was passed through the solution for a period of five minutes while maintaining the bath solution at about 43 C. The resulting chromium deposit was iridescent over approximately 74 millimeters of the electroplated surface when exposed to artificial light or to natural sunlight.

The following examples were carried out in a Hull Cell as described in Example 1 using the temperatures and conditions specified in Table II. The concentrations (in grams per literg./l. unless specified otherwise) are as given in Example 1 for each ingredient in the aqueous iridescent chromium plating bath and the results are summarized in the following table:

The following examples were carried out in a Hull Cell as described in Examples 1 through 6 using a bath composition containing 330 grams per liter of chromic acid (as CrO a sulfate concentration of 1.32 grams per liter and an acetic acid concentration of 16 grams per liter and an acetic acid concentration of 16 grams per liter. To this basic aqueous bath composition A were added the ingredients summarized in Table III. A 75 x 100 millimeter buffed brass Hull Cell panel containing a bright nickel deposit was plated in each example at a temperature of 43 C. for a time period of five minutes with a Hull Cell current of 8 amperes.

TABLE III Span of iridescence on Concentration experi- Exainple Material added to basic bath mental No. comp. A. g./l. m1.l1. panel 7 Methane sulfonic acid 8 Ethane sulfonic acid 85 9 1,2'ethane disulfonic acid 74 10 l-propane sulfonic acid 35 11 Ohloornethyl phosphonic 22 am 12 Methane hosphonic acid (di 22 Na salt 13 Methane phosphouic acid (di 16 Na salt 14 l-butane sulfonic acid (Con- 0 trol).

As can be seen by comparison of control Example 14 with Examples 1-13 iridescent is not obtained using 1- butane sulfonic acid.

Examples 15-27 were prepared by using a basic bath composition B containing 330 grams per liter of chromic acid (CrO 1.32 per liter of sulfate ion (as 50 and 4.2 grams per liter of methane sulfonic acid (except where noted otherwise) in combination with the acid materials in the amounts indicated in Table IV. For each example in Table IV, plating was carried out in a Hull Cell using a 75 x millimeter nickel plated brass panel, using a current of 8 amperes for five minutes at 43 C.

TABLE IV Span of iridescence on Concentration experi- Example mental N 0. Material panel 15 Acetic acid 82 Fluoroacetic acid 82 Propionic acid- 75 2-ch1oropropionie a 69 Chloroaeetic acid 63 Succinic acid (MSA=2.1 g./l.)- 60 Phosphate (MSA=1.4 g./l.) 41

(Succinic=40 g./l.). Adipic acid 33 23.- Tn'chloroacetic acid.. 27 24.. Sulionyl diacetie acid 24 25-. 1,3,5-tricarboxy pentaue 16 26 Boric acid 15 27 Tn'fiuoroacetic acid 15 Basic bath composition C was employed in Examples 28-31 summarized in Table V. Basic bath composition C contains 400 g./l. of chromic acid (as CrO 8 g./l. strontium sulfate, 20 g./l. acetic acid, and 4.2 g./l. of methyl sulfonic acid duluted to one liter with water to 7 form an equilibrium composition. Nickel plated brass panels were plated in a Hull Cell under the conditions noted in Table V.

TABLE V Span of iridescence on Plating experi- Example Temp., Current time (in mental 0. C. (in amps) minutes) panel TABLE VI Span of Concenlridesence Catalyst tratlon, on experi- Example No. anion g./l. mental panel Bath composition E containing 350 g./l. of chromic acid (Q03), 70 g./l. of fluosilicate anion, 20 g./l. of

acetic acid, and 2.8 g./1. of methyl sulfonic acid was used to plate a nickel plated brass panel measuring 75 x 1'00 millimeters in a standard Hull Cell using a current of 8 amperes for five minutes at a temperature of 43 to obtain an iridescent band millimeters wide.

Although this invention has been illustrated by reference to specific embodiments, modifications thereof which are clearly within the scope of the invention will be apparent to those skilled in the art.

We claim:

1. A process for preparing an iridescent chromium plated article which comprises passing current from an anode to a metal cathode through an aqueous chromium plating solution which contains at least one chromium compound providing hexavalent chromium metal ions for electroplating chromium, sulfate ions as catalysts and:

(a) as a first additive 2 grams per liter to 6 grams per liter of methyl sulfonic acid; and

(b) as a second additive 10 grams per liter to 25 grams per liter of acetic acid;

to produce an iridescent chromium surface on said article.

References Cited UNITED STATES PATENTS 2,745,801 5/1956 Blaine 204-51 2,750,334 6/1956 Brown 204--51 2,750,337 6/ 1956 Brown et a1. 204-51 2,846,380 8/1958 Brown 204-51 3,282,812 11/1966 Brown et a1. 204-51 3,310,480 3/1967 Brown 20451 3,311,548 3/1967 Brown et a1. 204-51 3,418,221 12/1968 Woods et a1. 2'04-51 GERALD L. KAPLAN, Primary Examiner 

